From Florida State University and the “correlation is not causation unless we want to blame climate change” department.

A stinging report: FSU research shows climate change a major threat to bumble bees

Research from a team of FSU scientists helps explain link between climate change and declining bumble bee populations

TALLAHASSEE, Fla. — New research from a team of Florida State University scientists and their collaborators is helping to explain the link between a changing global climate and a dramatic decline in bumble bee populations worldwide.

In a study published Friday in the journal Ecology Letters, researchers examining three subalpine bumble bee species in Colorado’s Rocky Mountains found that, for some bumble bees, a changing climate means there just aren’t enough good flowers to go around.

The team examined the bees’ responses to direct and indirect climate change effects.

“Knowing whether climate variation most affects bumble bees directly or indirectly will allow us to better predict how bumble bee populations will cope with continued climate change,” said FSU postdoctoral researcher Jane Ogilvie, the study’s lead investigator. “We found that the abundances of all three bumble bee species were mostly affected by indirect effects of climate on flower distribution through a season.”

As the global climate changes gradually over time, delicately poised seasonal cycles begin to shift. In the Rocky Mountains, this means earlier snowmelts and an extended flowering season.

On the surface, these climatic changes may seem like a boon to bumble bees — a longer flowering season might suggest more opportunity for hungry bees to feed. However, Ogilvie and her collaborators found that as the snow melts earlier and the flowering season extends, the number of days with poor flower availability increases, resulting in overall food shortages that are associated with population decline.

“When researchers think about flower effects on bees, they typically consider floral abundance to be the most important factor, but we found that the distribution of flowers throughout a season was most important for bumble bees,” Ogilvie said. “The more days with good flower availability, the more bees can forage and colonies can grow, and the bigger their populations become. We now have longer flowering seasons because of earlier snowmelt, but floral abundance has not changed overall. This means we have more days in a season with poor flower availability.”

Declining bumble bee populations globally have long been cause for alarm among conservationists, who see the buzzy pollinators as a bellwether for the malign effects of a changing climate.

Ogilvie said these most recent findings contribute to a growing body of evidence for the grave ecological consequences of climate change.

“Declining bumble bee populations should be a warning about the expansive detrimental effects of climate change,” Ogilvie said. “Bumble bees have annual life cycles, so their populations show responses to change quickly, and many species live in higher altitude and latitude areas where the change in climate is most dramatic. The effects of climate change on bumble bees should give us pause.”

The damage inflicted by climate change on global pollinator populations is of particular concern for scientists, as these species are crucial to agricultural productivity and the propagation of natural plant communities.

As researchers work toward a better understanding of climate change and its ecological effects, the link between pollinator health and shifting climate processes is becoming impossible to ignore.

“Pollinator species around the world have been declining, but we are still learning about what might be causing declines,” said FSU Professor of Biological Science Nora Underwood, a coauthor of the study. “Although not all species are influenced in the same way, I was excited to be part of this study because we now have long-term data that shows changing climate is influencing bumble bees.”

While this research helps to confirm the long-presumed connection between climate change and bumble bee population decline, Ogilvie said that the findings indicate a more difficult path for conservationists than previously anticipated.

“I’m afraid that this research shows conservation will be even more complicated than expected,” she said. “In addition to the response of the target species, our findings suggest that we should be considering how a species’ food resources might be responding to climate change. For bumble bees in particular, we need to make sure that they have enough flowers available during the entire season.”

Pollinators are in global decline and agricultural pesticides are a potential driver of this. Recent studies have suggested that pesticides may significantly impact bumblebee colonies—an important and declining group of pollinators. Here, we show that colony-founding queens, a critical yet vulnerable stage of the bumblebee lifecycle, are less likely to initiate a colony after exposure to thiamethoxam, a neonicotinoid insecticide. Bombus terrestris queens were exposed to field-relevant levels of thiamethoxam and two natural stressors: the parasite Crithidia bombi and varying hibernation durations. Exposure to thiamethoxam caused a 26% reduction in the proportion of queens that laid eggs, and advanced the timing of colony initiation, although we did not detect impacts of any experimental treatment on the ability of queens to produce adult offspring during the 14-week experimental period. As expected from previous studies, the hibernation duration also had an impact on egg laying, but there was no significant interaction with insecticide treatment. Modelling the impacts of a 26% reduction in colony founding on population dynamics dramatically increased the likelihood of population extinction. This shows that neonicotinoids can affect this critical stage in the bumblebee lifecycle and may have significant impacts on population dynamics.

Bees play a vital role as pollinators in both agricultural and natural systems1,2,3,4. However, there is increasing concern about the state of wild bee populations. Nearly 10% of European bee species are currently considered threatened5 and bumblebees are declining on a global scale5,6,7,8,9. The cause of these declines is thought to be a combination of factors, particularly habitat loss10, parasites and diseases11,12,13, invasive species14, and climate change15,16. Pesticide use is also considered a major threat to wild bees17,18,19,20, and both laboratory21,22,23,24,25,26, semi-field27,28,29,30,31,32,33 and field studies34,35,36have found negative impacts of pesticides on bumblebee behaviour, reproduction and colony success. However, information on the impacts of pesticides on the key life history stages of wild bees is still lacking. Bumblebees, like solitary bees, have an annual lifecycle whereby reproductive females (queens) initiate a colony in the spring37. Bumblebee queens are functionally solitary at this stage and do not have a colony to buffer them from environmental stress. Success depends entirely on the queen’s survival and ability to initiate a colony and, as such, this represents a critical but vulnerable period in the lifecycle. Although bumblebee queens are likely to be exposed to a range of pesticides throughout their lifecycle, particularly when foraging in the early spring on flowering crops such as OSR, to date there has been no research on the impacts of pesticides on founding queens and their ability to initiate a colony. Rundlöf et al.34 found that neonicotinoid treatment of OSR crops resulted in a lack of brood-cell building in solitary bees, but the mechanism remained unexplored. Negative impacts of neonicotinoids on the reproduction of the honeybee Apis mellifera queen have also been found38,39, but honeybee colonies are perennial and the way in which this relates to the annual cycle in bumblebees remains unknown. However, given these results, it is vital that we understand the potential impacts of pesticides on bumblebee queens40,26 and the resultant implications for wild populations.

We examined the impact of thiamethoxam (a neonicotinoid insecticide) exposure on colony-founding bumblebee (Bombus terrestris) queens. Neonicotinoids are the most widely used class of pesticide in the world41 and thiamethoxam is one of three neonicotinoids currently under a European Union usage moratorium for flowering, bee-attractive crops. Neonicotinoids have been implicated in the decline of wild bees20, butterflies42 and other taxa43. A range of regulations on the use of neonicotinoids have also recently come into force in North America. Therefore, research on the risks to beneficial insects associated with exposure to these compounds has important global policy implications.

In addition to the potential threat from pesticide exposure, bumblebee queens are faced with a range of environmental stressors that can reduce their survival and fitness. Before initiating a colony in the spring, queens must first survive hibernation over winter, during which time they can lose up to 80% of their fat reserves44, which may make them vulnerable to additional stress. Little is known about the overwintering survival of bumblebee queens in the wild, but studies in the laboratory have shown that a range of factors, such as pre-hibernation weight45,46, hibernation duration46 and the genotype of the queen and her mate47,48, can be important. Furthermore, exposure to parasites and pesticides can also impact hibernation survival40 and parasites have been shown to affect the post-hibernation success of queens. For example, Crithidia bombi, a prevalent trypanosome parasite of bumblebees, has a context-dependent impact on its queen host49. Under laboratory conditions, parasitized queens lost up to 11% more mass during hibernation and had up to a 40% reduction in fitness compared with uninfected queens49.

In natural environments, bumblebee queens face not only potential pesticide impacts, but also other simultaneous environmental stressors. To reflect this, we investigated the effects of thiamethoxam exposure on B. terrestris queens and tested for interactions with two natural environmental stressors: infection with the parasite C. bombi and variation in hibernation duration. To extrapolate our results to field populations, we used a Bayesian framework to assess their implications for population sustainability.